This invention relates generally to radiometric imagers, and, more particularly, to radiometric imagers that determine the frequency spectrum of a two-dimensional scene using a line source or linear array antenna and fanbeam inversion.
There are numerous practical applications for high-resolution radiometric imaging in aerospace, meteorological, oceanographic and astronomical fields. Radiometers are particularly well suited for the mapping of terrestrial, planetary and oceanic features, the measurement of atmospheric water vapor, rain and sea surface temperature, and the assessment of hydrographic phenomena and surface conditions beneath clouds or rain.
One prior microwave radiometer for providing a two-dimensional image of a scene using a line source antenna that scans the scene with a series of fanbeams is described in a copending and commonly-assigned U.S. patent application, Ser. No. 607,869, filed May 7, 1984, in the names of Carl A. Wiley and Charles R. Edelsohn, and entitled "Microwave Radiometer Using Fanbeam Inversion." This type of radiometer is referred to as "SPINRAD," which is an acronym for spinning radiometer. One embodiment disclosed in the copending application includes a mechanically-scanned pillbox antenna, while another embodiment includes an electronically-scanned, rotatable linear array antenna. In both cases, the antenna signals are suitably processed to reconstruct a single radiometric image of the scene being scanned.
Many applications for radiometric imaging require knowledge of the frequency spectrum of each segment of the scene being scanned. In the past, this has been achieved using a line source antenna whose beam direction does not change over the required frequency range, i.e., a non-frequency-dispersive antenna. A bank of bandpass filters divides the antenna output signal into a number of separate frequency bins, and a set of registered images of the scene is then produced using the data contained in the various frequency bins. Many line source antennas used in such systems are bulky and cumbersome in structure, such as the Rotman lens or planar reflector (i.e., pillbox). Other line source antennas, such as the standing-wave array, are small and light as compared to lenses and reflectors, but have unduly limited bandwidths.
It should, therefore, be appreciated that there is a need for a radiometric imager that can provide a spectral image of a scene over a wide frequency range, without being unduly bulky or cumbersome and without having an unduly limited frequency range. The present invention fulfills this need.